The lymphatics normally transport fluids and proteins against net hydrostatic pressure and protein gradients. Lymph is moved through the involvement of intrinsic and extrinsic lymphatic pumps and valved vessels. Lymphatics use phasic contractions and extrinsic compressions to generate flow, while tonic contractions alter resistance. Under normal conditions this intrinsic pump has been shown to be of primary importance in the generation of lymph flow within many lymphatics. Lymphatic muscle exhibits strong/phasic contractions, much higher shortening velocities and different intracellular calcium dynamics, when compared with smooth muscles. Unfortunately little information is known about the molecular mechanisms that are responsible for these characteristics. The central hypothesis of this proposal is that the regulatory mechanisms modulating the contraction dynamics of lymphatic muscle encompass the characteristics of both striated and smooth muscle. To test this hypothesis, the following specific aims are proposed: 1) To determine the distribution of contractile and regulatory proteins in lymphatic muscle; 2) to define the functional roles of thick filament regulatory pathways in phasic and tonic lymphatic muscle contraction; and 3) to determine the functional roles of thin filament regulatory pathways in lymphatic muscle contraction. We will use rat mesenteric and thoracic duct lymphatic vessels for the molecular studies. We will use isolated/cannulated vessels from mesenteric and thoracic duct lymphatics, to study the contractile characteristics of lymphatics. Force and calcium measurements will be conducted in the thoracic duct ring preparations (both intact and skinned) to determine the calcium sensitivity and cooperativity mechanisms of lymphatics. We expect to find a unique combination of contractile machinery in the lymphatic muscle. We predict that the phasic and tonic contractions of lymphatic vessels will reflect the inherent nature of the contractile and regulatory proteins that exist in this system. These studies will provide some of the first determinations of the unique mechanisms underlying lymphatic contractile behavior. The contractile mechanisms of lymphatics have not been known before and will significantly advance our understanding of the basis for the lymphatic vessel function.